Motor vehicle occupant restraint systems have been proposed in which a lap belt and/or a shoulder belt, referred to individually and collectively as "seat belts", are connected to a body of the motor vehicle through a belt tensioning and energy absorbing apparatus. The basic structural elements of such an apparatus include a stationary tube attached to the motor vehicle body, a piston slidable in the tube, a connecting member between the seat belt and the piston, a source of gas at elevated pressure, and an energy absorber. When sensors on the motor vehicle detect rapid deceleration characteristic of a collision, gas from the source of gas at elevated pressure is introduced into a pressure chamber between a head end of the stationary tube and the piston. The gas propels the piston through a tension stroke in a first direction during which the seat belt, through the connecting member, is pulled snugly around an occupant of the motor vehicle. At the conclusion of the tension stoke, deceleration of the motor vehicle relative to the occupant thrusts the occupant forward against the seat belt which thrust is transferred to the piston through the connecting member. The piston is propelled by the occupant's thrust in a second direction opposite to the first direction through an energy absorbing stroke during which the energy absorber converts into work a fraction of the kinetic energy of the occupant. While the tension stoke of the piston is typically relatively short, the pressure chamber at the head end of the stationary tube must be relatively long in order to assure adequate clearance for the energy absorbing stroke of the piston. Otherwise, the energy absorbing stroke of the piston could be prematurely terminated by engagement on the head end of the stationary tube. Because the volume of the pressure chamber is thus relatively large, a substantial volume of gas at elevated pressure is required to propel the piston through its tension stoke. A belt tension and energy absorbing apparatus according to this invention is more economical to manufacture than similar prior apparatuses because the volume of the pressure chamber is relatively smaller and requires less gas at elevated pressure to propel the piston through its tension stoke.